They are some what more valuable. I think the reason that they are not more valuable is they are so rare that most people, even Mustang people do not know what a HCS is.
With only 251 made and only a handful sold each year,consequently not lots of them on the big auctions or eBay so people just do not know what they are let alone pay big bucks for one. Say Shelby everyone knows the car and thinks big dollars. Interestingly
A 68 Shelby GT350 has a J code 302 same as a Mustang GTCS or HCS yet has a value just under 100K.
Marty